A water-based ink containing an aqueous dispersion of polymer particles containing a hydrophobic dye (hereinafter referred to as “dispersion of a dye-containing polymer”) has some characteristics such as water resistance and sharpness which are characteristics of the dispersion of a dye-containing polymer, and vividness, good color reproducibility and high optical density which are characteristics of the dye-based ink. As an example of a water-based ink in which the dispersion of a dye-containing polymer as described above is used, there have been known a colorant in which a vinyl polymer is impregnated with a hydrophobic dye as disclosed in Japanese Patent Laid-Open No. 54-058504; a dispersion of a dye-containing polymer, in which a water-dispersible polyester is impregnated with a water-insoluble dye as disclosed in Japanese Patent Laid-Open No. 07-34023; and a dispersion of a dye-containing polymer, in which a graft copolymer is used as a water-insoluble polymer as disclosed in Japanese Patent Laid-Open No. 08-035081, Japanese Patent Laid-Open No. 08-048498 and Japanese Patent Laid-Open No. 09-024680.
However, in a water-based ink, in particular, a water-based ink for use in inkjet recording, a water-soluble organic solvent for controlling the permeability of ink on paper (hereinafter referred to as permeability controlling solvent), a surfactant and the like are used as well as a dispersion of a dye-containing polymer. As the permeability controlling solvent, generally, a glycol ether, 2-pyrrolidone, isopropanol and the like have been used. However, these solvents plasticize the water-insoluble polymer. Therefore, the resin is plasticized and swelled when these solvents are used in an ink. As a result, the aqueous dispersion is degraded, which results in problems of increase in the viscosity of ink, increase in the particle diameter of dispersion, and even sedimentation of the polymer due to aggregation in the worst case.
Also, in the conventional dispersions of a dye-containing polymer, when the solvent as described above is used in an ink, the hydrophobic dye incorporated in the dispersed resin is dissolved in the solvent and extracted to the liquid phase. Therefore, the conventional inks have a problem that the dye is separated out when they are stored for a long period of time. As for the surfactant, when a nonionic surfactant having a low HLB is used, such nonionic surfactant causes degradation of the dispersion as in the above case, and the same problems arise.